Guess that one has to wait until they finish up the Switch version, I assume. Or some parts of the team already started with the development. If you consider how big the team is by now, porting might not require all of them.

If they were to release the DLC before the Switch version came out, there would have probably been an uproar. I wonder if they have been working on the DLC simultaneously to the Switch version, meaning we may see the DLC shortly after the Switch version releases in a couple of weeks. They were very quiet about the Switch version, and then suddenly we were given a release date just around the corner -- the same sudden surprise may come with the DLC.

I think the DLC may come by/around April, one year after the game's original release date. I don't think we will be waiting into 2019; if we are, I'd be expecting a huge update, which I don't think is the intention.